The invention relates to a method for conditioning at least one surface of a dielectric substrate for electrolessly plating a conductive metal thereon.
The process of electrolessly plating a metal (copper) onto a dielectric substrate is described for example in U.S. Pat. No. Re. 28,042 to Rhodenizer. One of the steps in this process is the seeding or catalyzing of the substrate surface prior to deposition of the metal onto the surface. This step is necessary since dielectric substrates are non-conductive.
Among the more widely employed procedures for catalyzing a substrate is the treatment of the surface with a stannous chloride sensitizing solution and a palladium chloride activator. For instance, one method for catalyzing a dielectric substrate is exemplified by U.S. Pat. No. 3,011,920 to Shipley which includes sensitizing the substrate by first treating it with a solution of a colloidal metal, accelerating the treatment with a selective solvent to remove protective colloids from the sensitized dielectric substrate and then electrolessly depositing a metal coating on the sensitized substrate; for example, with copper from a solution of a copper salt and a reducing agent.
U.S. Pat. Nos. 4,008,343 to Cohen et al and 3,562,038 to Shipley, Jr. et al also relate to methods for catalytically activating a surface with solutions containing Pd and Sn salts.
There have also been suggestions in the patent literature of treating substrates with certain surfactants. In particular, U.S. Pat. No. 4,301,190 to Feldstein suggests a pre-treatment of a substrate with an "adsorption modifier" to enhance the attachment to the substrate of a non-noble metal catalyst. Certain surfactants, hydride oxide sols and certain complexing agents are suggested as "adsorption modifiers".
U.S. Pat. No. 3,563,784 to Innes et al suggests a method of pretreating nonconductors for plating including a step of treating the surface with certain monofunctional surfactants, rinsing, and then activating either by a "two-step stannous chloride-palladium chloride treatment or the one-step acid, tin-palladium hydrosol treatment."
U.S. Pat. No. 3,684,572 to Taylor relates to a method of plating non-conductors including steps of treating the surface of the non-conductors with certain quaternary amine monofunctional or single charged surfactants after etching and before catalyzing the surface.
U.S. Pat. No. 3,573,937 to Drotar et al relates to a process for plating a non-conductive substrate. Reference is made therein to a step of rinsing the substrate with certain detergents before sensitizing with stannous chloride and activating with palladium chloride.
U.S. Pat. Nos. 3,515,649 to Hepfer; 3,877,981 to Arnold; and 3,930,072 to Wilks are of interest in showing the use of surfactants in plating processes in steps prior to the deposition of a catalyst.
U.S. Pat. No. 4,008,343 to Cohen et al suggests a process for electroless deposition wherein the catalytically prepared surface is rinsed with an aqueous acid solution with a pH of less than 1.5. The acid used is hydrochloric acid, perchloric acid, or nitric acid.
U.S. Pat. No. 3,491,922 to Wilson describes the application of a cationic film forming resin, specifically, melamine-formaldehyde resins, polyalkylene-amines, alkylated-methylol-melamines, triazine-formaldehyde and urea formaldehyde resins, onto substrate surfaces to be subsequently plated.
In copending application Ser. No. 398,140 and now U.S. Pat. No. 4,478,883 to Bupp et al assigned to the assignee of the present application a method for conditioning surfaces of dielectric substrates for the electroless plating is disclosed in which a copolymer of a non-reactive backbone chain of polyacrylamide to which are attached short chains of a tetra-alkyl-ammonium compound, with multiple charge functionality dissolved in a diluted inorganic acid, such as hydrochloric acid, is applied to the substrate surface prior to the activation.
Especially if the last of the cited methods is used, a very homogeneous metallization with excellent adhesion to the substrate is obtained. However if circuit lines are to be formed additively on the substrate surface (i.e., by forming a photoresist mask on the activated surface prior to plating), there is still the problem of residual resist (resist blistering) on the substrate surface after the developing step and the plated circuit lines tend to widen close to the substrate surface (line tailing).
Resist blistering can cause interruption in metal lines and line tailing shorts between adjacent metal lines.